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Thread: Multiconductor Cable Ampacity

  1. #11
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    Why do you have to use a cable?
    They say I shot a man named Gray and took his wife to Italy
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  2. #12
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    Quote Originally Posted by Dennis Alwon View Post
    Why do you have to use a cable?
    To preclude the possibility of voltage unbalance, due to inadvertent, unequal separation of individual phase wires,at pump motor input.

  3. #13
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    Quote Originally Posted by Sahib View Post
    To preclude the possibility of voltage unbalance, due to inadvertent, unequal separation of individual phase wires,at pump motor input.
    Huh??
    Bob

  4. #14
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    Quote Originally Posted by Dennis Alwon View Post
    Why do you have to use a cable?
    See post #5 where I asked the same question. You can get a supply house to twist the conductors into a bundle for you if you want.
    Bob

  5. #15
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    Is this 1-1/4 RGS actually used as a raceway or is it just a sleeve? Fill requirements don't apply to a sleeve, but are worth some consideration when there is significant length to them and how difficult it may be to get the cable in there.

    If 75C terminations are involved a 100 HP 460 volt motor must have 2/0 copper minimum conductor. You didn't say what the voltage is.

    My motor slide calculator does show 1 AWG if it were a 575 volt motor, but you did mention you need 124 ampacity which is NEC FLA for 460 volt motor, NEC requires minimum ampacity of 125% of that though in most applications so that puts required conductor ampacity at 155. Adjustments if required can start at the 90 C rating though.
    I live for today, I'm just a day behind.

  6. #16
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    Thanks for the comments. Here are a few responses.
    • The motor will run at 480V 3 phase.
    • I don't have to use a cable. But it gives me 53% of the internal area of the conduit, as opposed to 40%.
    • The RGS runs up from a gutter about 12 inches. It turns left via an elbow thingy (I don't know its name), and continues out of the range of my photo.
    • I think I could use the conduit as an EGC.
    • The selection of 100 HP is a design choice by the mechanical engineer. I asked him yesterday what the impact might be if I said he had to use a smaller motor. He was not pleased.
    • The power source (i.e., the other end of the RGS run) is a very old unit substation. I doubt it would have, or could get, 90C terminations.
    • I haven't looked into compact conductors yet.
    • I am told the run is closer to 650 feet than 500 feet. I would need a 2/0 just to keep VD below 3% anyway.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  7. #17
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    Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

    With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

    Any comments?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  8. #18
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    Quote Originally Posted by charlie b View Post
    Thanks for the comments. Here are a few responses.
    • The motor will run at 480V 3 phase.
    • I don't have to use a cable. But it gives me 53% of the internal area of the conduit, as opposed to 40%.
    • The RGS runs up from a gutter about 12 inches. It turns left via an elbow thingy (I don't know its name), and continues out of the range of my photo.
    • I think I could use the conduit as an EGC.
    • The selection of 100 HP is a design choice by the mechanical engineer. I asked him yesterday what the impact might be if I said he had to use a smaller motor. He was not pleased.
    • The power source (i.e., the other end of the RGS run) is a very old unit substation. I doubt it would have, or could get, 90C terminations.
    • I haven't looked into compact conductors yet.
    • I am told the run is closer to 650 feet than 500 feet. I would need a 2/0 just to keep VD below 3% anyway.
    My handy dandy chart says that a 480V 100 HP motor requires conductors with an ampacity of 124*125% or 155 Amps. That would give you a 2/0. Even if you can use an allowance for lower duty cycle that would only get you to a 1/0 conductor.

    A 600V motor only needs 99*125% worth of ampacity or 124 Amps. A #1 wire would do.

    No reason you could not splice on some larger conductors before terminating it at the old substation.
    Bob

  9. #19
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    Quote Originally Posted by charlie b View Post
    With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job.
    Quote Originally Posted by petersonra View Post
    Even if you can use an allowance for lower duty cycle that would only get you to a 1/0 conductor.
    How's come we are so far apart on this calculation?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

  10. #20
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    Aside: Is "RMC" the current and more appropriate name for what I have been calling "RGS"?
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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